A conventional apparatus calculates differences of luminance between respective pixels of a background image and respective pixels of a present image, and then, detects whether or not a detection target is present in a monitoring area based upon a total number of pixels which own luminance differences. As a result, in a case where brightness of the monitoring area when the background image is photographed is different from that when the present image is photographed, detecting precision as to the detection target is lowered. Accordingly, in order to avoid that the detecting precision as to the detection target is lowered due to the brightness differences in the conventional object detecting apparatus, brightness of the monitoring area is monitored, and the background image is updated according to the monitored brightness (refer to Patent Document 1).    Patent Document 1: JP 11-261994 A